Three-dimensional image measuring device

ABSTRACT

A three-dimensional image measuring device which comprises a light source; a plane image forming unit for forming a plane image in a space in its depth direction on the basis of light emitted from the light source; a scanning unit for moving and scanning the plane image formed by the plane image forming unit in its depth direction; an object to be measured disposed in the space,where the plane image is formed; a light receiving unit for measuring strength of light scattered on a surface of the object as the plane image is moved and scanned and; a distance measuring unit for measuring the distance to the Object on the basis of the output of the light receiving unit, whereby a three-dimensional image with least reduced invisible area is easily and accurately measured in a short time without using the principle of triangulation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of Ser. No. 08/165,418filed on Dec. 13, 1993 and which was granted on Sep. 5, 1995 under U.S.Pat. No. 5,448,360.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a three-dimensional image measuringdevice which obtains three-dimensional coordinate positions of an objectwithout using triangulation.

2. Description of the Related Art

In an active measuring process, light is cast upon an object, thestrength of light scattered on the surface of the object is measured andcoordinate positions of the object are measured on the basis of themeasured strength values using the principle of triangulation.

This process, however, has the following problems:

1) The distance to the object is required to be measured on the basis ofgeometric arrangement of the light source, projection angle of the lightand light receiving points (triangulation) and the calculation would becomplicated.

2) Since the light source and the light receiving points cannot bearranged on the same axis, an invisible area would be produced.

Published unexamined Japanese patent application Hei 4-1509, entitled"Non-Contact Three-Dimensional Coordinate Measuring Process", disclosesa technique which measures without using the principle of triangulation.

This technique relates to a three-dimensional beam scanner capable offocusing a laser beam on any point within the measurement space. Thescanner is controlled such that the focus of the beam is at a positionon the surface of an object to be measured, at which time thethree-dimensional coordinate position of the object is obtained on thebasis of a tilt of the laser beam within the reference coordinates andthe focal length.

This conventional technique has the following problems:

1) Control is always required to be provided such that the focus of thebeam is at a position on the surface of the object.

2) In order to measure the distance with high accuracy, a lens isrequired which is very shallow in depth of focus, small in aberration,and large in aperture. If the object is at a distance of more thanseveral meters from the measuring device, high measurement accuracy isnot obtained.

Apart from the active measurement, there is a passive measurementprocess for measuring the three-dimensional coordinates of the object bymoving the lens toward the object in place of beam irradiation todetermine a blur in the image. In this process, a position in an imageto which the lens is focused is the focal length of the lens. Thus, thedistance from the lens position is determined.

Since, however, this passive measurement process is intended to effectthe distance measurement using the lens focus, a lens is required whichis very shallow in depth of focus, small in aberration, and large inaperture in order to measure the distance with high accuracy, as in thesecond conventional technique. If the subject is at a distance of morethan several meters from the measuring device, high measurement accuracyis not obtained.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problems. It is anobject of the present invention to provide a three-dimensional imagemeasuring device which is capable of measuring a three-dimensional imagewith a small invisible area easily with high accuracy in a short timewithout using the principle of triangulation.

To achieve the above object, the present invention provides athree-dimensional image measuring device which comprises a light source;plane image forming means for forming a plane image in a space in itsdepth direction on the basis of light emitted from the light source;scanning means for moving and scanning the plane image formed by theplane image forming means in its depth direction; an object to bemeasured disposed in the space where the plane image is formed; lightreceiving means for measuring the strength of light scattered on asurface of the object as the plane image is moved and scanned; anddistance measuring means for measuring a distance to the object on thebasis of the output of the light receiving means.

In this invention, a plane image is formed in a space where the objectis disposed. The image is moved and scanned in the depth direction. Thatportion of the plane image of the object to be cut scatters lightstrongly compared to the remaining portion of the image. The scatteredlight is received by the light receiving means and the distances to thecorresponding portions of the object are measured on the basis of theoutputs from the light receiving means. In more detail, as the planeimage is scanned, the distance to the plane image present when thestrength of the scattered light or contrast is maximum is calculated foreach of light receiving elements of the light receiving means and thecalculated distances are considered as the distances to the appropriatepixels of the object.

Thus, according to the present invention, a plane image is formed in thespace where the object is disposed. The plane image is moved and scannedin the depth direction to scatter light from the object, the scatteredlight is received, and the distance to the object is measured.Accordingly, only a single easy scan brings about a three-dimensionalimage in the space. Since the light source and the light receivingpoints are disposed on substantially the same axis, no invisible area isproduced. If the plane image focusing means is a hologram, anaberration-free, shallow-depth of focus bright image is obtained. Thus,high accuracy distance measurement is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of the present invention.

FIG. 2 shows the rotational elements of the hologram of the firstembodiment.

FIGS. 3(a) and 3(b) illustrate multi-exposure of the hologram.

FIG. 4 shows a second embodiment of the present invention.

FIG. 5 shows a third embodiment of the present invention.

FIG. 6 shows division of a shutter of the third embodiment.

FIG. 7 shows divided exposure of the hologram of the third embodiment;and

FIG. 8 shows a fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with respect toembodiments shown in the accompanying drawings.

FIG. 1 shows an embodiment of the present invention. The laser beam Lemitted from a laser light source 1 is diverged by a lens 2 to enter asreference light into a hologram 3, which is rotated by a rotating motor4 and which has an observation window 5 in the center thereof, as shownin FIG. 2.

In this case, the hologram 3 is subjected to multi-exposure such thatplane images are imaged at corresponding different depth distances d inaccordance with the respective rotational angles of the hologram 3.

As shown in FIG. 3(a), in order to create such multiple-exposurehologram 3, a plane object J is disposed at a position and irradiatedwith the reference beam when the hologram 3 is at a rotational angle δ.The plane object J is disposed at a position different from the previousposition by changing the rotational angle 6 of the hologram 3 andirradiated with the reference beam. Such exposure is iterated for aplurality of times.

As shown in FIG. 3(b), by rotating the hologram 3, which has beensubjected to such multi-exposure, while being irradiated with thereference beam, plane images J' are imaged at corresponding differentdepth positions.

Since the hologram 3 which has been subjected to multi-exposure isirradiated with the reference beam from the source 1, as shown in FIG.1, the plane image J' is imaged at a position corresponding to therotational angle of the hologram 3.

Thus, if an object 6 to be measured is disposed in a space where theplane image J' is formed, that portion of the object 6 cut by the planeimage J' strongly scatters the light.

The scattered light is caused to enter through the observation window 5and the focusing lens 7 into a two-dimensional image sensor 8, which iscomposed of a plurality of light receiving elements arranged in atwo-dimensional array. The image sensor 8 outputs an opto-electricconversion signal depending on the scattered light entering into arespective one of the light receiving elements. The output signal fromthe image sensor 8 enters into a distance measuring unit 9, whichmeasures the distance to the object 6.

In more detail, in FIG. 1, the reference light from the source 1 entersthe hologram 3, which has been subjected to multi-exposure, while thehologram 3 is being rotated (for example, one rotation) by the motor 4to move and scan the plane image J' in the depth direction.

The distance measuring unit 9 sequentially samples the outputs of therespective light receiving elements of the image sensor 8 as the imageis moved and scanned. A plane image imaging distance d when the strengthof scattered light or contrast is maximum or at a peak period iscalculated for each of the light receiving elements (pixels) and thecalculated distance d is considered as the distance of the appropriatepixel of the object. Since the distance d corresponds to the distancebetween the position at which the plane image J is disposed at themulti-exposure and the position of the hologram 3, each distance d isdetermined for each rotational angle of the hologram 3.

In this way, the distance image of the object 6 is obtained for each ofthe pixels.

FIG. 4 shows a second embodiment of the present invention.

In this embodiment, light sources k (each of which includes a laserlight source 1 and lens 2) are disposed at different positions. Byselecting those light sources sequentially, the hologram 3 is subjectedto multi-exposure such that plane images J' are formed at differentpositions in a depth direction of the halogen 3. The light source k maybe moved in place of the sequential selection of the light sources.Alternatively, light sources may be selectively switched instead.

For example, the hologram may be beforehand subjected to multi-exposuresuch that the plane image J'-1 is imaged by the light source k-1 and theplane image J'-n is imaged by the light source k-n.

By sequentially one by one lighting up the sources k-1 to k-n atdifferent positions to the hologram 3, which has been subjected tomulti-exposure, plane images J' different in depth are imagedsequentially.

The structure of the device for light reception is similar to that ofthe embodiment shown in FIG. 1. The respective outputs of the lightreceiving elements of the image sensor 8 are sequentially sampled incorrespondence to the scan of the source k. A plane image imagingdistance d present when the strength of scattered light or contrast ismaximum or at a peak period during the scan is calculated for each ofthe light receiving elements (pixels) and the resulting distance d isconsidered as the distance of the appropriate pixel of the object 6. Inthis way, the distance image of the object 6 is obtained for each pixelas in the previous embodiments.

FIG. 5 shows a third embodiment of the present invention. In thisembodiment, a liquid crystal shutter 10 is provided in front of thehologram 3. As shown in FIG. 6, the shutter 10 is composed of aplurality of concentric areas 10-a, . . . , 10-d. Voltages (electricfields) applied to respective areas of the liquid crystal are controlledsuch that the areas open sequentially one by one. The shutter 10 has anobservation window 11 as in the hologram 3.

In this case, as shown in FIG. 7, the hologram 3 is beforehand subjectedto partial exposure such that plane images are formed at differentdistances d in concentric areas 3-a, . . . , 3-d similar to those of theshutter 10.

For example, one area of the shutter 10 is opened and reference light iscasted on the hologram 3 in a state where a plane object is disposed ata position. Another area of the shutter 10 is then opened and thereference light is casted on the hologram 3 in a state where the planeobject is disposed at a position different from the previous positionjust mentioned. By iterating such exposure process a plurality of times,the hologram 3 which are subjected to partial exposure is formed.

By casting the laser light L from the source 1 on the resulting hologram3 and sequentially selecting the open areas of the liquid crystalshutter 10, plane images are sequentially imaged at different positionsin the depth direction of the hologram.

Thus, in this case, the respective outputs of the light receivingelements of the image sensor 8 are sampled in correspondence to theselection of the open areas of the liquid crystal shutter 10, the planeimage imaging distances d present when the strength of the scatteredlight or contrast is maximum or at a peak period due to the switching ofthe shutter 10 is calculated for each of the light receiving elements(pixels) and the calculated plane image imaging distances d isconsidered as the distance to the appropriate pixel of the objects 6. Inthis way, the distances image is obtained for each of the pixels, assame with the previously described embodiment.

While in the above embodiments,the area is divided into concentricareas, the shape of the divided areas is freely determined. While in theembodiments the liquid crystal shutter is used, any other type ofshutter may be used as long as the shutter has the same function.

FIG. 8 shows a fourth embodiment. In this embodiment, the laser beam Lfrom the source 1 is amplified by a lens 2, then collimated by a lens12, and the resulting collimated rays of light are casted as referencelight to the hologram 3.

In this case, the hologram 3 is beforehand once subjected to exposuresuch that when the reference light is casted on the hologram 3, a planeimage is formed at a position where the distance between that positionof the formed plane image and the position of the hologram 3 is apredetermined distance do. In this case, the hologram 3 is arranged soas to be moved by a moving actuator 13 in the depth direction (shown byan arrow M) with the angle θ between the surface of the hologram 3 andthe reference light L being constant.

Thus, when the actuator 13 causes the hologram 3 to be moved andscanned, the plane image is moved and scanned in the depth direction (inthe direction of the arrow M) such that the plane image is formed at aposition where the distance between that position of the plane image andthe position of the hologram 3 is do at all times.

Thus, the distance measuring unit 9 sequentially samples the respectiveoutputs of the light receiving elements of the image sensor 8 as thehologram 3 is moved and scanned, calculates the sum D of the distance ofmovement of the hologram 3 (from a reference position) and the planeimage imaging distance do present when the strength of the scatteredlight or the contrast is maximum or at a peak period during the movementand scan of the hologram 3 for each of the light receiving elements(pixels) and the calculated distance D is considered as the distance tothe appropriate pixel of the object 6. In this way, the distance imageof the object 6 is obtained for the respective pixels.

While in the above-mentioned respective embodiments the laser beam isused as the light source, pure monochromatic light or white light may beused instead. While the lens 2 is used as the beam diverging means, ahologram which has the same function may be used.

What is claimed is:
 1. A three-dimensional image measuring devicecomprising:a hologram adapted for single exposure such that whenreproduced, parallel light having a predetermined incident angle isirradiated, a plane light image is formed at a position located at apredetermined distance from the hologram in a depth direction; a lightsource for irradiating reproduced light on the hologram; parallel lightconverting means for converting the light from the light source intoparallel light and irradiating the parallel light at the predeterminedincident angle to the hologram; hologram moving and scanning means forimaging the parallel light image at a plurality of different positionsextending in the moving and scanning direction by moving and scanningthe hologram in a predetermined direction in such a manner that an anglebetween the parallel light and the hologram is kept at the predeterminedincident angle; an object adapted to be measured being disposed in aspace where the plane light image is formed; light receiving meansincluding a plurality of two-dimensional-arranged light receivingelements for sequentially measuring the strength of scattered light ofthe plane light image imaged on a surface of the object as the hologramis moved by the hologram moving and scanning means; and distancemeasuring means for measuring the distance to the object based on anoutput of each light receiving element of the light receiving means. 2.The three-dimensional image measuring device according to claim 1wherein the hologram has an opening at a part of its region, and thelight receiving means is disposed at a position where the lightreceiving means receives the scattered light of the plane light imagewhich is imaged on the surface of the object through the opening of thehologram.
 3. The three-dimensional image measuring device according toclaim 1 wherein the distance measuring means obtains for each of thelight receiving elements of the light receiving means a distance fromthe hologram to the plane light image when the strength or contrast ofthe scattered light is maximum and carries out a calculation with eachof the obtained distance being considered as a distance of each pixel ofthe object.